1. Field of the Invention
The present invention relates to a lightweight optical glass of fluoro-phosphate that has specific gravity not more than 3.6 with the optical constants converting refractive index nd=1.42-1.50 and Abbe number .nu.d=80-97.
The fluoro-phosphate optical glass under the present invention has significant optical importance as it has extraordinary ray dispersibility and makes it possible to revise the secondary spectrum at the time of lens designing, and also offers high value use as lens material for large diameter camera and television camera lenses.
2. Related Background Art
Crystalline fluorspar (CaF.sub.2) has been known as the optical material having extraordinary ray dispersibility and light weight, and its optical constants and specific gravity are as follows:
nd=1.4388 PA1 vd=95.3 PA1 Sg=3.18
Fluorspar, however, has disadvantages in connection with cleavage and fragility. Moreover, in the face of recent increasing reguirements of large diameter lenses, it is difficult to obtain large lumps of fluorspar of homogenous quality and they are very expensive if found.
Thus, various kinds of fluoro-phosphate optical glass as optical material to replace fluorspar have been developed (ref. Japanese Publication No. 54-34768, Japanese Laid-Open Patent Appln. No. 1-270537 and Japanese Publication No. 57-44618), and some of them are available in the market under the nomenclatures of FK01, FCD1, FK02, FCD10 and CaFK95.
However, those conventional glasses have large specific gravity as compared with fluorspar and are not quite desirable for use as optical material for optical apparatus having large diameter lenses.
Also, an invention under Japanese Publication No. 57-44618 requires addition of Si as the indispensable ingredient to improve stability against devitrification and increase viscosity in the molding process. However, if Si is contained in the glass, it often causes devitrification with Si as crystalline nuclear in the pressing process for forming a large size lens, making it very difficult to obtain large diameter lenses.